The field of the present invention is polyamide powders for the coating of metals.
The invention is particularly concerned with methods for the production of coating compositions, for the melt film coating method, based on polyamides and adhesion improving additives based on diisocyanates.
The state of the prior art of producing polamide powders may be ascertained by reference to U.S. Pat. Nos. 3,476,711; 3,900,607; 3,927,141; 3,966,838; 4,143,025; 4,195,162; 4,273,919; and 4,334,056; British Pat. Nos. 535,138; 688,771 and 1,408,975; West German Patent Document C 1,272,286; and the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Ed., Vol. 16 (1968), under the section "Polyamide (Plastics)", pages 88-105, particularly page 92-polylauryllactam (nylon-12), and polyundecanamide (nylon-11), page 101 Solution Processes, and Powder Processing, pages 101-102, the disclosures of which are incorporated herein by reference.
The flame spraying and fluidized bed coating of nylon on a metal base is disclosed in U.S. Pat. No. 3,203,822.
The isophoronediisocyanates capped with lactams and acetonoximes used in the present invention are prepared as disclosd in U.S. Pat. No. 3,822,240.
The dimerisation and trimerisation of isophoronediisocyanate is disclosed in GB-PB 1,153,815 and GB-PS 1,386,399.
It is known to use pulverulent polyamide based coating compositions to prepare varnish type metal coatings. The coating is implemented by the melt-film coating method, that is, by the fluidized bed procedure, the flame spray method or by the electrostatic coating process.
The powders are prepared either by the precipitation method by precipitating the polyamide from solutions or by the grinding procedure by grinding the polyamide granulate or scrap at low temperatures.
Furthermore it is known to add adhesion improving additives to the powders. For instance, coating compositions made of plastics containing carbonamide groups are described in West German Patent Document C 1,271,286 for the preparation of coatings by the fluidized bed or flame spray methods, which are characterized by an adhesion-improving addition of a monomeric or oligomeric compound having amino-, epoxy-, carbonyl-, hydroxy-, mercapto-, nitrile-, or isocyanate-groups.
In Example 1 of West German Patent Document C 1,271,286 this addition consists of a dimerized toluylene-2,4-diisocyanate with two free isocyanate groups per molecule.
When an object heated to 320.degree. to 350.degree. C. is coated with such a polyamide powder in a fluidized bed, a well-adhering cross-linked coating is in fact achieved, however it is present as a foam and, therefore, unfit as corrosion protection for a metal surface.
British Pat. No. 1,408,975 describes a method for preparing coatings on metal objects by electrostatically dusting powders made of polyamides and/or copolyamides containing a phenol and then heating to a temperature above the melting points of the powders. An adduct in the form of a solid phenol having a melting point of 30.degree. to 300.degree. C. with mono-, di-, or polyisocyanates is added to the powders and heating is performed for 3 to 25 minutes to 220.degree. to 250.degree. C. maximum so that no cross-linking of the polyamides takes place. This method accordingly is applicable only by electrostatically coating within the stated temperature range. However, as regards fluidized bed coatings which require substrate temperatures that are higher by 100.degree. to 150.degree. C. than the 220.degree.-250.degree. C. range, powders so applied produce cross-linked foams providing no corrosion protection to the melt surface as in West German Patent Document C 1,271,286.
The state of the art, therefore, does not permit, as a rule, the use of the melt film process for the preparation of well adhering coatings based on polyamides.